DIVERSITY AND PALEOECOLOGY IN THE NEOTROPICS: TAKING A CLOSER LOOK AT MARINE BIOTAS OF THE FAMOUS GATUN FORMATION, PANAMA

The late Miocene (Tortonian) Gatun Formation of Panama is regarded as one of the most diverse marine fossil assemblages in the Caribbean, with more than 500 species, 300 genera, and 86 families of molluscs documented. The formation is often regarded as exemplar among diverse Late Neogene Neotropical assemblages, and indeed is used as a baseline in studies investigating the effects of the closure of the Isthmus of Panama. A number of workers have undertaken exhaustive sampling of these macrofaunas, but none have attempted to understand the fine-scale spatial and temporal variations in the composition of the Gatun fauna

High-resolution faunal inventories were conducted from over 700 stratigraphic horizons in the Gatun Formation, at more than 20 sections and boreholes in the Canal Basin of central Panama. Over 200 species were captured using this surveying approach, representing mostly widespread or common, and typically macroscopic species. This data permits quantitative paleoecological analyses of fossil assemblages through the stratigraphy of the Gatun Formation. Detrended correspondence analysis reveals a stratigraphic pattern of variation in the faunal composition of individual horizons as well as differences between sampling sites. In addition to their utility as tools of correlation between sections, curves of DCA axis scores provide a means to detect relative shifts in water depth and sedimentation through the stratigraphy and among localities. Overall the Gatun reveals both coarse- and fine-scale temporal variations in paleobathymetry, reflecting long-term shifts in basin subsidence and accommodation space that is potentially overprinted by fifth- or sixth-order glacio-eustatic cycles. Local species richness varies somewhat through the formation, being influenced by stratigraphic variation in the density of fossils and outcrop weathering. The high total diversity of the Gatun Formation is partly the result of excellent fossil preservation, extensive outcrops, and subtle turnover in species composition between assemblages.